Pistachio shell biochar as a reinforcing filler in short Turkish hemp fiber composites: a path toward sustainable materials

This study investigated the potential of pistachio nut shells (PNS) as a biochar filler in short Turkish hemp (STH) fibre natural fibre-based polymeric composites. PNS were made into biochar through pyrolysis and incorporated into a polyester resin matrix reinforced with STH fibres at varying concentrations (5–25 wt. %). The composites were fabricated using compression moulding and characterised for their mechanical, thermal, acoustic, and water absorption properties. The results showed that the incorporation of PNS biochar significantly enhanced its tensile strength of 47.36 MPa, flexural strength of 82.88 MPa, impact strength of 28.68 kJ/m², and Shore-D hardness of 82.49, with an optimal concentration of 15 wt. %. The thermal conductivity increased to 0.185 W/m·K, while the water absorption in seawater and distilled water was minimised to 3.8% and 5.7%, respectively. The sound absorption properties were also improved, with a maximum noise reduction coefficient (NRC) of 0.30. However, at high biochar concentrations (> 15 wt. %), it led to particle clustering and void formation, resulting in reduced mechanical and acoustic properties. Scanning electron microscopy (SEM) revealed that these biochar composites had minimal void formation and strong interfacial bonding between the biochar and hemp fibres for the optimal 15 wt. % compositions. This highlights the feasibility of using PNS biochar as a sustainable filler in natural fibre composites, offering a promising alternative to synthetic materials with enhanced properties and reduced environmental impact.